To find the molecular formula, you must first find the molecular mass from the data given. Then, take the molecular mass and divide it by the mass of HC3H5O3. Approximate that to the nearest whole number and then multiply the empirical formula by that number to get your molecular formula.

Try to figure it out and post your work so that we can help you further.

since to reach the end point titration means to reach the point at which the number of moles of NaOH is equal to the number of moles of HC3H5O3, the number of moles of HC3H5O3 is 0.011.
0.011 mol*(90.8 g HC3H5O3/mol) = 0.991 g HC3H5O3 (the mass of HC3H5O3).

Think about it this way. It took .011 moles of NaOH to reach the end point ( .017 L * (.65 mol/L) ). At the end point, there are equal amounts of moles of the acid and base. Therefore, 1.0 g * (1 mol / X grams) = .011 moles. Now, solve for X and you have your molecular mass. Then continue as before.

is it wrong to use the empirical formula to find the molecular mass?
i would use the molecular formula to find the molecular mass, but the molecular formula is what is in question.

what then is the mass of HC3H5O3? is it 90.08?

i previewsly said the molecular weight (i meant molecular mass) = 1.00794 g H + 3*(12.0107 g C) + 5*(1.00794 g H) + 3*(15.9994 g O) = 90.8 g HC3H5O3. the number is wrong. it is not 90.8 g. it is 90.08 g. also, it is not the molecular mass. is it the mass of HC3H5O3?